Power outlet ground integrity and wriststrap monitor circuit

Abstract

A plug-mounted wriststrap detector and ground monitor includes hot, neutral and ground input terminals which are connected to corresponding terminals of a power outlet when the circuit is plugged in. A jack is provided to plug a wriststrap into the circuit to provide electrostatic protection to the user. Means are provided to detect that the user is wearing the wriststrap and that the wriststrap is properly connected to the circuit as a function of whether an a.c. signal of sufficient amplitude is detected coupled to the circuit by the wriststrap and/or user. Lamps and an audible alarm warn the user if the wriststrap is not properly connected. A precision ground integrity circuit is provided to detect whether an unsafe ground condition exists and to alert the user accordingly. The ground integrity circuit uses a constant current source and voltage comparator to determine that the ground impedance does not exceed a predetermined limit.

What is claimed is: 
     
       1. A self-contained wriststrap detector circuit comprising: a housing containing the circuit;   hot, neutral and ground input terminals connected to and extending from said housing for insertion in a power outlet;   means on said housing for removably connecting a wriststrap to said circuit;   an impedance in said housing coupling said wriststrap connecting means to said ground input terminal for developing a predetermined voltage only when a user of the wriststrap is electrostatically grounded and when the housing input terminals are properly connected to said power outlet;   means coupled to said impedance for detecting said predetermined voltage and generating a first output signal representing a safe condition only when said wriststrap is properly connected to said circuit and said user, and a second output signal representing an unsafe condition only when said wriststrap is improperly connected either to said circuit or said user;   indicator means in said housing for indicating the safe and unsafe conditions to said user;   means coupled between said signal generating means and said indicator means for activating the safe and unsafe condition indication in response to said first and second output signals; and   a low frequency band pass filter for removing unwanted signals, said band pass filter having an input connected to said wriststrap impedance means and an output coupled to said signal generating means.   
     
     
       2. A wriststrap detector circuit according to claim 1 wherein said bandpass filter has a low frequency cutoff less than 50 hertz. 
     
     
       3. A wriststrap detector circuit according to claim 2 wherein said bandpass filter has a high frequency cutoff less than 1000 hertz. 
     
     
       4. A monitor for testing electrical power outlets comprising: a plug-in module having a hot input terminal and at least one ground reference input terminal connected to and extending therefrom for insertion in and electrically connectable with corresponding terminals of a power outlet;   means in the plug-in module for generating a low level constant current;   an adjustable resistance in the plug-in module coupling the constant current in series with the ground reference input terminal to cause a threshold voltage level across the adjustable resistance, the voltage level across the adjustable resistance increasing above a reference voltage only when an unsafe ground condition exists;   means in the plug-in module coupled to the adjustable resistance for sensing the voltage across the adjustable resistance and generating an output signal when it exceeds the reference voltage because an unsafe ground condition occurs; and   means in the plug-in module responsive to the voltage sensing means output signal for indicating if the unsafe ground condition exists when the plug-in monitor is plugged into the outlet.   
     
     
       5. A ground monitor according to claim 4 wherein said voltage sensing means comprises: a voltage comparator having one input connected between said adjustable resistance and said constant current generating means; and   a second input of said comparator connected to said reference voltage, said comparator generating said output signal only when the voltage across the adjustable resistance exceeds said reference voltage.   
     
     
       6. A ground monitor according to claim 5 wherein said indicating means includes visual and audible alerts and further includes switching transistors which turn on and off in response to said output signals from said voltage comparator to control actuation of said visual and audible alerts. 
     
     
       7. A ground monitor according to claim 6 wherein said comparator produces a first output signal representing said unsafe ground condition when said voltage across said adjustable resistance exceeds said reference voltage set by adjustment of said resistance, and a second output signal representing said safe ground condition when said voltage across said adjustable resistance does not exceed said reference voltage. 
     
     
       8. A self-contained wriststrap detector circuit comprising: hot, neutral and ground input terminals connectable to a power outlet;   means for removably connecting a user wriststrap to said circuit;   impedance means coupled between said wriststrap connecting means and the neutral input terminal for developing a predetermined a.c. signal voltage only when the user of the wriststrap is electrostatically grounded to the neutral input terminal and the circuit is properly connected to the power outlet;   means coupled to the impedance means for rectifying the a.c. signal voltage to obtain an input d.c. voltage;   a d.c. reference voltage;   a first comparator circuit for receiving the input d.c. voltage and the d.c. reference voltage and generating a first output signal only when the wriststrap is properly connected to both said circuit and to said user and a second output signal when the wriststrap is improperly connected either to said circuit or said user; and   indicator means coupled to the first comparator circuit for continuously indicating the proper wriststrap connection only when the first output signal is generated and continuously indicating an improper wriststrap connection only when the second output signal is generated.   
     
     
       9. A wriststrap detector circuit according to claim 8 wherein said indicator means further includes means for visually indicating said proper wriststrap connection as a safe condition to the user. 
     
     
       10. A wriststrap detector circuit according to claim 8 wherein said indicator means includes: first and second indicator lamp means;   a switching transistor being connected in series with said first indicator lamp and in parallel with said second indicator lamp; and   said first comparator circuit first output signal turning the switching transistor off to cause the second lamp to indicate said proper connection and the second output signal of said first comparator circuit turning the switching transistor on to cause the first lamp to indicate said improper wriststrap connection.   
     
     
       11. A wriststrap detector circuit according to claim 10 further comprising a low frequency bandpass noise filter circuit having an input coupled to said wriststrap connecting means and an output coupled to an input of said rectifying means. 
     
     
       12. A wriststrap detector circuit according to claim 11 wherein said bandpass filter has a low frequency cutoff less than 50 hertz. 
     
     
       13. A wriststrap detector circuit according to claim 12 wherein said bandpass filter has a high frequency cutoff less than 1000 hertz. 
     
     
       14. A wriststrap detector circuit according to claim 13 further comprising: means for monitoring the effective impedance of the ground terminal when it is connected to the power outlet to detect an unsafe ground condition;   said monitoring means including a constant current source and an adjustable resistance in series with the constant current source and the ground input terminal to develop a variable voltage across the adjustable resistance;   a second reference voltage;   a second comparator circuit comparing the voltage across the adjustable resistance with the second reference voltage and generating an output signal indicative of a safe and unsafe ground condition; and   said indicator means being responsive to the second comparator circuit output signals to indicate said safe and unsafe ground conditions, respectively.   
     
     
       15. A wriststrap detector circuit according to claim 14 wherein said indicator means includes an audible alarm. 
     
     
       16. A wriststrap detector circuit according to claim 14 wherein said second comparator circuit output signal comprises at least first and second distinct output signals, one of which corresponds to said safe ground condition and the other corresponds to said unsafe ground condition. 
     
     
       17. A wriststrap detector circuit according to claim 16 wherein the indicator means includes first and second lamp means which respectively indicate the safe and unsafe ground conditions, and means coupled to the second comparator circuit for controlling operation of said lamp means in response to said second comparator circuit first and second output signals. 
     
     
       18. A wriststrap detector circuit according to claim 17 wherein said controlling means is a switching transistor connected in parallel with one of said lamp means.